AbstractsEarth & Environmental Science

The use of concrete in disposal facilities to store low- and intermediate-level nuclear waste requires that the durability of this material is optimal for the lifetime of these repositories. One of the most important processes that put such durability at risk is the attack of concrete by water with low mineral content and neutral pH, altering its microstructure (decalcification, dissolution of cement phases, increased porosity and loss of barrier properties ). Therefore, to yield reliable estimations of the durability of cement-based materials we need to increase our knowledge of the reactivity of these materials. Within this context, in this Thesis, the dissolution kinetics of the calcium silicate hydrate (C-S-H) -the main binding phase in all cement-based systems- and the processes responsible for mortar alteration have been studied. The dissolution kinetics of C-S-H gel was determined by flow-through experiments. Demineralized water caused C-S-H dissolution and changes in the composition of the solutions. It was observed that the C-S-H gel dissolved incongruently when the Ca/Si ratio was high and congruently as the Ca/Si ratio decreased to the tobermorite-like stoichiometric Ca/Si ratio of 0.83. A dissolution rate law for C-S-H gel with Ca/Si ratio equal to 0.83 was proposed based on the dissolution rates normalized to the final BET surface area. Additionally, reactive transport modeling of the changes in aqueous chemistry allowed the fitting of the rate constants for C-S-H with Ca/Si ratio ranging from 1.7 to 0.83. Solid examination by SEM-EDX and EPMA showed some variability of the Ca/Si ratios of the analyzed particles, suggesting the existence of compositional domains with variable Ca/Si ratios. 29Si MAS-NMR spectra showed an increase in polymerization of the reacted C-S-H, and also the formation of Si-rich domains in some cases, mainly under slow flow conditions. Additionally, the changes in the microstructure of the dissolving C-S-H gel were characterized by small angle neutron scattering (SANS). The SANS data were fitted using a fractal model. The SANS specific surface area (ST) tended to increase with time up to 31 days. Thereafter, it diminished as the C-S-H gel dissolved. The rest of the fitted parameters (particle diameter (Do), fractal exponents (DV and DS), etc.) reflect a more open and ordered nanostructure of the gel. To study the altered mortar (64% cement I42.5R / SR and 36% fly ash) two types of experiments were performed. On one hand, Column experiments using mortar fragments of ¿ 2 mm in size were performed with CO2-free atmosphere at room temperature. Variation of the chemical composition and the inspection of the mortar fragments by means of optical inspection, SEM and XRF-before and after the experiments- allowed interpretation of the occurring dissolution and precipitation reactions. The aqueous chemical data were modeled using the CrunchFlow reactive transport code in which the obtained dissolution rate law for the C-S-H gel and the associated kinetic parameters were incorporated. On the…